Radio communication system, base station, and control method thereof

ABSTRACT

A radio communication system  1  according to an embodiment comprises: a pico cell base station PeNB provided in a communication area of a macro cell base station MeNB and having transmission power lower than transmission power of the macro cell base station MeNB. A resource division ratio is determined with respect to radio resources to be usable as a PDSCH, indicating a ratio of macro cell unusable PDSCH resources and macro cell usable PDSCH resources. The radio resources are assigned to a radio terminal connected to the macro cell base station MeNB, out of the macro cell usable PDSCH resources determined according to the determined resource division ratio. The resource division ratio is determined according to expected throughputs of each of cell edge terminals of the macro cell base station MeNB and the pico cell base station PeNB.

TECHNICAL FIELD

The present invention relates to a radio communication system applied toa heterogeneous network, a base station, and a control method of thebase station.

BACKGROUND ART

As a next-generation system for performing high speed communication withhigh capacity, as compared with the 3rd-generation and 3.5th-generationcellular radio communication systems operated at present, there are LTE(Long Term Evolution), which is standardized by 3GPP (3rd GenerationPartnership Project) of a standardization body, and LTE Advanced whichis a sophisticated version of LTE.

In a downlink of an LTE system (including LTE Advanced), a base stationtransmits user data to a radio terminal using a data transmissionchannel called PDSCH (Physical Downlink Shared Channel). In addition,the downlink indicates communication toward the radio terminal from thebase station, and an uplink indicates communication toward the basestation from the radio terminal.

Furthermore, in LTE Advanced, it is discussed to provide a heterogeneousnetwork, that is a network in which a low power base station (so-calleda pico cell base station, a femto cell base station, or a relay node) islocated in a communication area of a high power base station (so-calleda macro cell base station). In the heterogeneous network, it is possibleto distribute a load of the high power base station to the low powerbase station.

However, since it is general that a radio terminal is connected to abase station, which has the highest received power of a radio signalamong a plurality of base stations, it is probable that a connectionopportunity of the radio terminal to a low power base station with lowtransmission power is reduced in the heterogeneous network.

In this regard, there has been proposed a technique of expanding acoverage (a communication area range) of a low power base station bycontrolling a radio terminal to be connected to the low power basestation even though received power from the low power base station isnot highest (for example, refer to Non Patent Literature 1).

CITATION LIST Non Patent Literature

-   Non Patent Literature 1: 3GPP R1-101506 “Importance of Serving Cell    Association in Heterogeneous Networks” February, 2010.

SUMMARY OF THE INVENTION

However, when radio resources used as data transmission channels betweenneighboring base stations overlap each other, since a data transmissionchannel of one base station receives interference from a datatransmission channel of the other base station, it is probable that itis not possible to normally receive user data through the datatransmission channel of the one base station.

Particularly, in the technique of expanding the coverage of the lowpower base station in the heterogeneous network, since it is highlyprobable that a data transmission channel of the low power base stationreceives significant interference from a data transmission channel ofthe high power base station, the aforementioned problem becomes morecritical.

Therefore, it is an object of the present invention to provide a radiocommunication system, a base station, and a control method of the basestation, with which it is possible to reduce inter-base stationinterference in a heterogeneous network and to improve the throughput ofthe entire system.

In order to solve the aforementioned problem, the present invention hasfollowing characteristics. First, the characteristic of the radiocommunication system according to the present invention is summarized asfollows. A radio communication system (radio communication system 1)comprising: a high power base station (e.g., macro cell base stationMeNB); and a low power base station (e.g., pico cell base station PeNB)provided in a communication area of the high power base station andhaving transmission power lower than transmission power of the highpower base station, comprises: a division ratio determination unit(division ratio determination unit 123 or division ratio determinationunit 225) that determines a resource division ratio with respect toradio resources to be used as a specific downlink channel (e.g., PDSCH)by the high power base station, the resource division ratio indicating aratio of first radio resources (e.g., macro cell usable PDSCH resourcesor macro cell normal power PDSCH resources) and second radio resources(e.g., macro cell unusable PDSCH resources or macro cell normal powerPDSCH resources), the transmission power of the low power base stationfor second radio resources being limited to be lower than that for thefirst radio resources, wherein the division ratio determination unitdetermines the resource division ratio according to expected throughputof a high power-side deteriorated terminal having a deterioratedreception state, and expected throughput of a low power-sidedeteriorated terminal having a deteriorated reception state, the highpower-side deteriorated terminal being a radio terminal connected to thehigh power base station, the low power-side deteriorated terminal beinga radio terminal connected to the low power base station. Here, thespecific downlink channel is, for example, a downlink data transmissionchannel (PDSCH in the LTE system). However, the specific downlinkchannel may also be a downlink control information transmission channel(PDCCH in the LTE system) as well as such a data transmission channel.Furthermore, the low power base station is, for example, a pico cellbase station or a femto cell base station. However, the low power basestation may also be a relay node or the like as well as the pico cellbase station or the femto cell base station.

In accordance with the radio communication system according to theaforementioned characteristic, with respect to the radio resources to beused as the specific downlink channel of the high power base station,the first radio resources and the second radio resources are provided,the transmission power of the high power base station for the secondradio resources being limited to be lower than the first radioresources. Since interference from the high power base station for thesecond radio resources is reduced, it is possible to improve thethroughput of the low power-side deteriorated terminal by assigning thesecond radio resources to the low power-side deteriorated terminal.Furthermore, by determining the resource division ratio according to thethroughput of the high power-side deteriorated terminal and thethroughput of the low power-side deteriorated terminal, it is possible,for example, to equalize the throughput of cell edge terminals of eachof the high power base station and the low power base station, resultingin the improvement of the throughput of the entire system.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic, thedivision ratio determination unit determines the resource division ratiosuch that the expected throughput of the high power-side deterioratedterminal is equal to the expected throughput of the low power-sidedeteriorated terminal.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic, theexpected throughput of the high power-side deteriorated terminal isdetermined on the basis of a load level of the high power base stationand expected unit throughput corresponding to a reception quality levelof the high power-side deteriorated terminal, and the expectedthroughput of the low power-side deteriorated terminal is determined onthe basis of a load level of the low power base station and expectedunit throughout corresponding to a reception quality level of the lowpower-side deteriorated terminal.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic, aload level of the high power base station indicates the number of radioterminals connected to the high power base station, and a load level ofthe low power base station indicates the number of radio terminalsconnected to the low power base station.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic, thelow power-side deteriorated terminal includes a first low power-sidedeteriorated terminal to which the first radio resource is assigned, anda second low power-side deteriorated terminal to which the second radioresource is assigned, and throughput of the low power-side deterioratedterminal is determined on the basis of the load level of the low powerbase station, expected unit throughput corresponding to a receptionquality level of the first low power-side deteriorated terminal, andexpected unit throughput corresponding to a reception quality level ofthe second low power-side deteriorated terminal.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic,when the load level of the high power base station is set as #UEperM,the expected unit throughput corresponding to the reception qualitylevel of the high power-side deteriorated terminal is set as TP_(MUE),the load level of the low power base station is set as #UEperP, theexpected unit throughput corresponding to the reception quality level ofthe first low power-side deteriorated terminal is set as TP_(PUE1), andthe expected unit throughput corresponding to the reception qualitylevel of the second low power-side deteriorated terminal is set asTP_(PUE2), the division ratio determination unit determines a ratio α ofthe first radio resource to radio resources to be used as the specificdownlink channel by using the calculation equation:

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack\mspace{670mu}} & \; \\{\alpha = \frac{{{TP}_{{PUE}\; 2} \cdot \#}{UEperM}}{{{{TP}_{{MUE}\;} \cdot \#}{UEperP}} + {{\left( {{TP}_{{PUE}\; 2} - {TP}_{{PUE}\; 1}} \right) \cdot \#}{UEperM}}}} & \;\end{matrix}$or a calculation equation equivalent to the calculation equation.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic,when a plurality of the low power base stations are provided in thecommunication area of the high power base station, the division ratiodetermination unit uses an average value or a maximum value of loadlevels of the low power base stations as the load level of the low powerbase station.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. The radio communicationsystem according to the aforementioned characteristic, furthercomprises: a resource assignment unit (resource assignment unit 124)that assigns a radio resource to a radio terminal connected to the highpower base station, wherein the first radio resource is a radio resourceusable by the high power base station (e.g., macro cell usable PDSCHresources), the second radio resource is a radio resource unusable bythe high power base station (e.g., macro cell unusable PDSCH resources),and the resource assignment unit assigns a radio resource of the firstradio resources, which are determined according to the resource divisionratio determined by the division ratio determination unit, to the radioterminal connected to the high power base station.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. The radio communicationsystem according to the aforementioned characteristic, furthercomprises: a resource assignment unit (resource assignment unit 124)that assigns a radio resource to a radio terminal connected to the highpower base station, wherein the first radio resource is a radio resourcein which transmission power of the high power base station is notlimited (e.g., macro cell normal power PDSCH resources), the secondradio resource is a radio resource in which the transmission power ofthe high power base station is limited (e.g., macro cell low power PDSCHresources), and the resource assignment unit assigns a radio resource ofthe first radio resources and the second radio resources, which aredetermined according to the resource division ratio determined by thedivision ratio determination unit, to the radio terminal connected tothe high power base station.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic,when a plurality of the low power base stations are provided in thecommunication area of the high power base station, the division ratiodetermination unit determines the division ratio on the basis of anaverage value or a maximum value of load levels of each of the low powerbase stations.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic, thespecific downlink channel is a data transmission channel fortransmitting user data to the radio terminal.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic, thesecond radio resources are at least a part of frequency bands of allfrequency bands of a downlink, and the first radio resources correspondto remaining frequency bands of all the frequency bands of the downlinkexcept for the partial frequency bands.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic,each of the partial frequency bands and the remaining frequency bands isan integer times of a frequency unit by which the radio terminalmeasures a reception quality level.

Another characteristic of the radio communication system according tothe present invention is summarized as follows. In the radiocommunication system according to the aforementioned characteristic, thesecond radio resources are at least a part of a time range of a dataregion for transmitting user data to the radio terminal in acommunication time frame of a downlink, and the first radio resourcesare a remaining time range of the data region except for the part of atime range.

The characteristic of a base station according to the present inventionis summarized as follows. A base station comprises: a division ratiodetermination unit that determines a resource division ratio withrespect to radio resources to be used as a specific downlink channel,the resource division ratio indicating a ratio of first radio resourcesand second radio resources, transmission power for the second radioresources being limited to be lower than that for the first radioresources, wherein the division ratio determination unit determines theresource division ratio according to expected throughput of adeteriorated terminal being a radio terminal connected to the basestation and having a deteriorated reception state, and expectedthroughput of a deteriorated terminal being a radio terminal connectedto a neighboring base station and having a deteriorated reception state.

The characteristic of a control method of a base station according tothe present invention is summarized as follows. A control method of abase station, comprises: a step of determining a resource division ratiowith respect to radio resources to be used as a specific downlinkchannel, the resource division ratio indicating a ratio of first radioresources and second radio resources, transmission power for the secondradio resources being limited to be lower than that for the first radioresources, wherein in the step of determining, the resource divisionratio is determined according to expected throughput of a deterioratedterminal being a radio terminal connected to the base station and havinga deteriorated reception state, and expected throughput of adeteriorated terminal being a radio terminal connected to a neighboringbase station and having a deteriorated reception state.

The present invention can provide a radio communication system, a basestation, and a control method of the base station, with which it ispossible to reduce inter-base station interference in a heterogeneousnetwork and to improve the throughput of the entire system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining the overview of an LTE systemaccording to the first embodiment to the third embodiment.

FIG. 2 is a diagram illustrating a communication frame configuration inthe LTE system.

FIG. 3 is a schematic configuration diagram of a radio communicationsystem according to the first embodiment.

FIG. 4 is a diagram for explaining the ICIC according to the firstembodiment.

FIG. 5 is a block diagram illustrating the configuration of the macrocell base station according to the first embodiment.

FIG. 6 is a block diagram illustrating the configuration of the picocell base station according to the first embodiment.

FIG. 7 is an operation sequence diagram illustrating the operation ofthe radio communication system according to the first embodiment.

FIG. 8 is a block diagram illustrating the configuration of the macrocell base station according to the second embodiment.

FIG. 9 is a block diagram illustrating the configuration of the picocell base station according to the second embodiment.

FIG. 10 is an operation sequence diagram illustrating the operation ofthe radio communication system according to the second embodiment.

FIG. 11 is a diagram for explaining the ICIC according to the thirdembodiment.

FIG. 12 is a diagram for explaining the case of time-dividing the PDSCHresources.

FIG. 13 is a diagram for explaining another case of time-dividing thePDSCH resources.

DESCRIPTION OF THE EMBODIMENTS

The first embodiment to the third embodiment and the other embodimentsof the present invention will be described. In the drawings of each ofthe embodiments, the same or similar parts are provided with the same orsimilar reference signs.

[Overview of LTE System]

Before a description of the first embodiment to the third embodiment,the overview of an LTE system will be described while focusing on thecontent associated with the present invention.

FIG. 1 is a diagram for explaining the overview of an LTE system. Asillustrated in FIG. 1, a plurality of base stations eNB constituteE-UTRAN (Evolved-UMTS Terrestrial Radio Access Network). Each of theplurality of base stations eNB forms a cell that is a communication areawhere a service should be provided to a radio terminal UE.

The radio terminal UE is a radio communication device carried by a user,and is also called as “User Equipment”. The radio terminal UE isconnected to a base station eNB, which has the highest received power ofa reference signal (RSRP: Reference Signal Received Power) among theplurality of base stations eNB. However, as well as the RSRP, anotherreception quality indexes such as SNR (Signal-to-Noise ratio) may beused.

Each base station eNB is able to communicate mutually via an X2interface which is a logical communication channel to provide inter-basestation communication. Each of the plurality of base stations eNB isable to communicate with EPC (Evolved Packet Core), specifically, MME(Mobility Management Entity)/S-GW (Serving Gateway), via an S1interface.

In radio communication between the base station eNB and the radioterminal UE, an OFDMA (Orthogonal Frequency Division Multiple Access)scheme is applied as a downlink multiplexing scheme and an SC-FDMA(Single-Carrier Frequency Division Multiple Access) scheme is applied asan uplink multiplexing scheme. Furthermore, an FDD (Frequency DivisionDuplex) scheme or a TDD (Time Division Duplex) scheme is applied as aduplex scheme.

FIG. 2( a) is frame configuration diagram illustrating a downlink radioframe configuration when the FDD scheme is used. FIG. 2( b) is a frameconfiguration diagram illustrating the configuration of a downlinksub-frame.

As illustrated in FIG. 2( a), the downlink radio frame consists of 10downlink subfames, and each downlink subfame consists of two downlinkslots. Each downlink subfame has a length of 1 ms and each downlink slothas a length of 0.5 ms. Furthermore, as illustrated in FIG. 2( b), eachdownlink slot includes seven OFDM symbols in a time axis direction (timedomain), and includes a plurality of resource blocks (RB) in a frequencyaxis direction (frequency domain). Each RB includes 12 subcarriers.

As illustrated in FIG. 2( b), the downlink subfame includes twocontinuous downlink slots. A zone of up to three ODFM symbols from thehead of a first downlink slot in the downlink subfame is a controlregion constituting a radio resource used as PDCCH (Physical DownlinkControl Channel) for transmitting control information. The controlinformation corresponds to uplink and downlink scheduling information(that is, information of an assignment radio resource) and the like. Azone of remaining ODFM symbols of the downlink subframe is a data regionconstituting a radio resource used as PDSCH (Physical Downlink SharedChannel) for transmitting user data. The radio terminal UE decodes thecontrol information transmitted by the PDCCH, thereby designating userdata transmitted by the PDSCH.

First Embodiment

Next, the first embodiment of the present invention will be described.In the first embodiment, a description will be provided for an exampleof an embodiment of a heterogeneous network arrangement in which a picocell base station PeNB as a low power base station is provided in acommunication area (a macro cell) of a macro cell base station MeNB as ahigh power base station.

In the first embodiment below, the description will be given in theorder of (1) Configuration of radio communication system, (2)Interference control by resource division, (3) Configuration of macrocell base station, (4) Configuration of pico cell base station, (5)Operation of radio communication system, and (6) Effect of firstembodiment.

(1) Configuration of Radio Communication System

FIG. 3 is a schematic configuration diagram of a radio communicationsystem 1 according to the first embodiment.

As illustrated in FIG. 3, the radio communication system 1 includes amacro cell base station MeNB (a high power base station or a largeoutput base station), a radio terminal MUE connected to the macro cellbase station MeNB, a pico cell base station PeNB (a low power basestation or a small output base station) provided in a macro cell MCformed by the macro cell base station MeNB and adjacent to the macrocell base station MeNB, and a radio terminal PUE connected to the picocell base station PeNB in a pico cell PC formed by the pico cell basestation PeNB. The macro base station MeNB and the pico cell base stationPeNB use a common frequency band. In the first embodiment, a pluralityof pico cell base stations PeNB 1 to PeNB 3 are provided in the macrocell MC. In addition, the pico cell PC formed by the pico cell basestation PeNB is appropriately called a “hot zone” hereinafter.

The pico cell base station PeNB (also called a hot zone node) is a lowpower base station having transmission power lower than that of themacro cell base station MeNB. Therefore, in the heterogeneous network,when employing a maximum received power reference (hereinafter, an RPreference) that is a connection destination selection reference in whichthe radio terminal UE selects and is connected to a base station eNBwith the highest RSRP, the coverage of the pico cell base station PeNBmay be small. Particularly, in a situation in which the position of thepico cell base station PeNB is near the macro cell base station MeNB,the coverage of the pico cell base station PeNB is significantly small,so that it is not possible to effectively utilize the pico cell basestation PeNB.

As a method for expanding the coverage of the pico cell base stationPeNB without an increase in the transmission power of the pico cell basestation PeNB, the following two methods can be mainly employed.

Firstly, instead of the RP reference for selecting a base station eNB,which transmits a radio signal with the highest RSRP, as a connectiondestination of the radio terminal UE, there is a method for selecting abase station eNB, which has the lowest propagation loss (path loss)between the base station eNB and the radio terminal UE, as theconnection destination of the radio terminal UE. In this way, forexample, since a base station eNB nearest the radio terminal UE isselected as the connection destination, it is possible to expand thecoverage of the pico cell base station PeNB. Such a connectiondestination selection reference is called a minimum path loss reference(hereinafter, a PL reference).

Secondly, there is a method in which a bias value (bias) is added toRSRP corresponding to the pico cell base station PeNB when the RSRPcorresponding to the pico cell base station PeNB is compared with RSRPcorresponding to the macro cell base station MeNB in the case in whichthe radio terminal UE is able to receive a radio signal from each of themacro cell base station MeNB and the pico cell base station PeNB. Thebias is given to the RSRP corresponding to the pico cell base stationPeNB, so that it is more likely that the RSRP after the offset exceedsthe RSRP corresponding to the macro base station MeNB. Thus, since thepico cell base station PeNB is preferentially selected as the connectiondestination, it is possible to expand the coverage of the pico cell basestation PeNB. Such a connection destination selection reference iscalled a Range Expansion reference (hereinafter, an RE reference). Thebias value is shared by a pair of the macro cell base station MeNB andthe pico cell base station PeNB. In addition, a difference between thetransmission power of the macro cell base station MeNB and thetransmission power of the pico cell base station PeNB (for example, 16dB) can be set as the bias value, so that the RE reference is aconnection destination selection reference equivalent to the PLreference.

In the first embodiment, the cell coverage of the pico cell base stationPeNB is assumed to be expanded according to the RE reference. Inaddition, an entity for selecting the connection destination of theradio terminal UE is, for example, the radio terminal UE when the radioterminal UE is in a standby state (an idle state), and a base stationeNB (a connection destination) when the radio terminal UE is performingcommunication (a connected state). In the connected state, since ameasurement value of the RSRP is periodically reported to the basestation eNB (the connection destination) from the radio terminal UE, thebase station eNB (the connection destination) is able to select a nextconnection destination of the radio terminal UE, and perform handover ofthe radio terminal UE to the next connection destination.

According to the RE reference, a pico cell base station PeNB1 forms anexpanded cell PC1, and radio terminals PUE1 are connected to the picocell base station PeNB1 in the cell edge of the expanded cell PC1.Furthermore, according to the RE reference, a pico cell base stationPeNB2 forms an expanded cell PC2, and radio terminals PUE2 are connectedto the pico cell base station PeNB2 in the cell edge of the expandedcell PC2. Moreover, according to the RE reference, a pico cell basestation PeNB3 forms an expanded cell PC3, and radio terminals PUE3 areconnected to the pico cell base station PeNB3 in the cell edge of theexpanded cell PC3.

Hereinafter, a radio terminal PUE located in the cell edge, that is, aradio terminal PUE having a deteriorated reception quality level iscalled a “pico cell-side deteriorated terminal PUE (a low power-sidedeteriorated terminal)”. Furthermore, a radio terminal MUE located inthe cell edge, that is, a radio terminal MUE having a deterioratedreception quality level is called a “macro cell-side deterioratedterminal MUE (a high power-side deteriorated terminal)”. In addition, asthe reception quality level, SINR (Signal-to-Interference and Noisepower Ratio) is exemplified.

The macro base station MeNB transmits user data to the radio terminalMUE using the PDSCH. The pico cell base station PeNB transmits user datato the radio terminal PUE using the PDSCH. When frequency bands of thesePDSCHs overlap each other, the PDSCHs of the macro cell base stationMeNB and the pico cell base station PeNB are interfered with each other.

In the state in which the coverage of the pico cell base station PeNB isexpanded, in the pico cell-side deteriorated terminal PUE, the receivedpower from the macro cell base station MeNB may be higher than thereceived power from the pico cell base station PeNB. In this case, thePDSCH of the pico cell base station PeNB receives significantinterference from the PDSCH of the macro cell base station MeNB, theradio terminal PUE is not able to receive (decode) user data.

(2) Interference Control by Resource Division

In the downlink of the heterogeneous network, when adding the biasaccording to the RE reference such that the coverage is expanded to belarger than the hot zone created according to the RP reference,interference power is larger than power of a desired signal according tothe difference between the transmission power of the macro cell basestation MeNB and the transmission power of the pico cell base stationPeNB. Thus, a radio terminal UE not optimal in terms of SINR isaccommodated in the hot zone. Since such a radio terminal UE basicallyreceives significantly strong interference from a macro cell basestation MeNB with high transmission power, SINR is significantly low. Inthis regard, in the first embodiment, interference control is performedby the following ICIC (ICIC: Inter-Cell Interference Coordination).

FIG. 4 is a diagram for explaining the ICIC according to the firstembodiment.

As illustrated in FIG. 4( a), PDSCH resources (corresponding to the dataregion illustrated in FIG. 2( b)) of the macro cell base station MeNBare frequency-divided and a part thereof is not used, so that the picocell-side deteriorated terminal PUE is able to use the unused part.PDSCH resources unusable by the macro cell base station MeNB are called“macro cell unusable PDSCH resources”, and PDSCH resources usable by themacro cell base station MeNB are called “macro cell usable PDSCHresources”. In the first embodiment, the macro cell usable PDSCHresources are at least a part of all resource blocks of a downlink, andthe macro cell unusable PDSCH resources are remaining resource blocks ofall the resource blocks of the downlink, except for the partial resourceblocks. Furthermore, in the first embodiment, the macro cell usablePDSCH resources correspond to first radio resources and the macro cellunusable PDSCH resources correspond to second radio resources.

As illustrated in FIG. 4( b), since the macro cell unusable PDSCHresource does not receive interference from the macro cell base stationMeNB, each pico cell base station PeNB assigns the macro cell unusablePDSCH resource to a pico cell-side deteriorated terminal PUE connectedthereto. In addition, since the radio terminal PUE periodically feedsback a measurement result of reception quality to the pico cell basestation PeNB as channel quality information (CQI), each pico cell basestation PeNB is able to preferentially assign a non-interference PDSCHresource to the pico cell-side deteriorated terminal PUE according tothe fact that CQI corresponding to the non-interference PDSCH resourceis good.

In the ICIC by such frequency division, instead of avoiding interferenceto the hot zone, the macro cell usable PDSCH resources usable by theradio terminal MUE connected to the macro cell base station MeNB arereduced. In this regard, for the improvement of characteristics by theexpansion of the coverage of the hot zone, a characteristic improvementeffect by load distribution needs to exceed loss due to the reduction ofusable resources by the frequency division. Furthermore, in order toreliably improve the throughput of the pico cell-side deterioratedterminal PUE, it is necessary to consider how low the SINR of the picocell-side deteriorated terminal PUE is (that is, how large the appliedbias value is).

In this regard, in the first embodiment, the macro cell base stationMeNB determines a resource division ratio according to expectedthroughput of the macro cell-side deteriorated terminal MUE and expectedthroughput of the pico cell-side deteriorated terminal PUE.Specifically, the macro cell base station MeNB determines the resourcedivision ratio such that the expected throughput of the macro cell-sidedeteriorated terminal MUE is equal to the expected throughput of thepico cell-side deteriorated terminal PUE.

The expected throughput of the macro cell-side deteriorated terminal MUEis determined on the basis of a load level of the macro cell basestation MeNB, and expected unit throughput corresponding to SINR of themacro cell-side deteriorated terminal MUE. The expected throughput ofthe pico cell-side deteriorated terminal PUE is determined on the basisof a load level of the pico cell base station PeNB, and expected unitthroughput corresponding to SINR of the pico cell-side deterioratedterminal PUE.

The load level indicates the number of radio terminals in a connectedstate. Thus, the load level of the pico cell base station PeNB indicatesthe number of radio terminals PUE connected to the pico cell basestation PeNB, and the load level of the macro cell base station MeNBindicates the number of radio terminals MUE connected to the macro cellbase station MeNB. When a plurality of pico cell base stations PeNB areprovided in the same macro cell as is the case with the example of FIG.1, an average value or a maximum value of the number of radio terminalsPUE that are connected to each pico cell base station PeNB is used asthe load level of the pico cell base station PeNB.

The expected unit throughput indicates throughput per one resourceblock, which is provided in a modulation and coding scheme (MCS)according to SINR. As the SINR is better, MCS with a larger modulationlevel is larger a lower coding rate is used, so that the expected unitthroughput is larger. Meanwhile, as the SINR is worse, MCS with asmaller modulation level and a higher coding rate is used, so that theexpected unit throughput is smaller.

In the first embodiment, the pico cell-side deteriorated terminal PUEincludes a first pico cell-side deteriorated terminal PUE to which themacro cell usable PDSCH resource is assigned, and a second picocell-side deteriorated terminal PUE to which the macro cell unusablePDSCH resource is assigned. Therefore, the expected throughput of thepico cell-side deteriorated terminal PUE is determined on the basis ofthe number of terminals connected to the pico cell base station PeNB,expected unit throughput corresponding to the first pico cell-sidedeteriorated terminal PUE, and expected unit throughput corresponding tothe second pico cell-side deteriorated terminal PUE.

When the number of terminals connected to the macro cell base stationMeNB is set as #UEperM, the expected unit throughput corresponding tothe SINR of the macro cell-side deteriorated terminal MUE is set asTP_(MUE), the number of terminals connected to the pico cell basestation PeNB is set as #UEperP, the expected unit throughputcorresponding to the SINR of the first pico cell-side deterioratedterminal PUE is set as TP_(PUE1), and the expected unit throughputcorresponding to the SINR of the second pico cell-side deterioratedterminal PUE is set as TP_(PUE2), the macro cell base station MeNBcalculates the ratio α (refer to FIG. 4( a)) of the macro cell usablePDSCH resources to radio resources to be used as PDSCH on the basis of acalculation equation expressed by the following:

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack\mspace{644mu}} & \; \\{\frac{{TP}_{{MUE}\;} \cdot \alpha}{\#{UEperM}} = \frac{{{TP}_{{PUE}\; 1} \cdot \alpha} + {{TP}_{{PUE}\; 2} \cdot \left( {1 - \alpha} \right)}}{\#{UEperP}}} & (1)\end{matrix}$In Math. 2, the left side corresponds to the expected throughput of themacro cell-side deteriorated terminal MUE, and the right side of theMath. 2 corresponds to the expected throughput of the pico cell-sidedeteriorated terminal PUE. By converting Equation (1), the followingEquation (2) is obtained.

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack\mspace{644mu}} & \; \\{\alpha = \frac{{{TP}_{{PUE}\; 2} \cdot \#}{UEperM}}{{{{TP}_{{MUE}\;} \cdot \#}{UEperP}} + {{\left( {{TP}_{{PUE}\; 2} - {TP}_{{PUE}\; 1}} \right) \cdot \#}{UEperM}}}} & (2)\end{matrix}$

The resource division ratio may be arbitrarily set. However, accordingto the LTE specifications, the resources are divided according to theresolution of CQI fed back from the radio terminal UE. That is, therespective frequency bands of the macro cell usable PDSCH resource andthe frequency band of the macro cell unusable PDSCH resource are aninteger times of a frequency unit by which the radio terminal UEmeasures reception quality (channel quality). The frequency unit iscalled a sub-band (Subband). When the frequency band of the macro cellunusable PDSCH resource is set as m (=1−α) and the frequency band of themacro cell usable PDSCH resource is set as n (=α), the number RB ofresource blocks of the macro cell usable PDSCH resource is expressed bythe following Equation (3).

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack\mspace{644mu}} & \; \\{{RB} = {{SubbandSize} \cdot \left\lfloor {N_{RB} \cdot \frac{n}{\left( {m + n} \right)} \cdot \frac{1}{SububandSize}} \right\rfloor}} & (3)\end{matrix}$

In this case, the SubbandSize indicates the size (resolution) of CQIthat is fed back, and the N_(RB) indicates the total number RB ofdownlink frequency bands.

(3) Configuration of Macro Cell Base Station

Next, the configuration of the macro cell base station MeNB will bedescribed. FIG. 5 is a block diagram illustrating the configuration ofthe macro cell base station MeNB according to the first embodiment.

As illustrated in FIG. 5, the macro cell base station MeNB includes anantenna 101, a radio communication unit 110, a control unit 120, astorage unit 130, and a network communication unit 140.

The radio communication unit 110, for example, is configured using aradio frequency (RF) circuit, a baseband (BB) circuit and the like, andperforms transmission/reception of a radio signal with the radioterminal MUE through the antenna 101. Furthermore, the radiocommunication unit 110 performs modulation of a transmission signal anddemodulation of a reception signal.

The control unit 120 is configured, for example, using a CPU, andcontrols various functions provided in the macro cell base station MeNB.The storage unit 130 is configured, for example, using a memory, andstores various types of information used for the control and the like ofthe macro cell base station MeNB. The storage unit 130 stores a biasvalue of each pico cell base station PeNB. The network communicationunit 140 performs inter-base station communication with another basestation using an X2 interface.

The control unit 120 includes a connection destination selection unit121, an information acquisition unit 122, a division ratio determinationunit 123, and a resource assignment unit 124.

The connection destination selection unit 121 selects a base station,which is a next connection destination of the radio terminal MUE, on thebasis of information (that is, a measurement report) of RSRP reportedfrom the radio terminal MUE connected to the macro cell base stationMeNB. In the case in which the radio terminal MUE receives a referencesignal from each of the macro cell base station MeNB and the pico cellbase station PeNB, when RSRP_(MeNB) corresponding to the macro cell basestation MeNB is compared with RSRP_(PeNB) corresponding to the pico cellbase station PeNB, the connection destination selection unit 121 appliesbias to the RSRP_(PeNB). When the biased RSRP_(PeNB) is higher than theRSRP_(MeNB), the connection destination selection unit 121 performshandover control such that a connection destination of the radioterminal MUE is switched to the pico cell base station PeNB.

The information acquisition unit 122 acquires the number #UEperM ofterminals connected to the macro cell base station MeNB, the expectedunit throughput TP_(MUE) corresponding to the SINR of the macrocell-side deteriorated terminal MUE, the number #UEperP of terminalsconnected to the pico cell base station PeNB, the expected unitthroughput TP_(PUE1) corresponding to the SINR of the first picocell-side deteriorated terminal PUE, and the expected unit throughputTP_(PUE2) corresponding to the SINR of the second pico cell-sidedeteriorated terminal PUE. The information acquisition unit 122 is ableto acquire the number #UEperM of terminals connected to the macro cellbase station MeNB using information managed by the resource assignmentunit 124. Furthermore, the information acquisition unit 122 is able toacquire the number #UEperP of terminals connected to the pico cell basestation PeNB using information on the number of connected terminals,which is received in the network communication unit 140 from the picocell base station PeNB. For the expected unit throughput TP_(MUE),TP_(PUE1), and TP_(PUE2), an expected value may be used or an actuallymeasured value may be used. In the case of using the expected value,each of the expected unit throughput TP_(MUE) and TP_(PUE2) may be usedas Shannon capacity in case of the SINR=0 [dB], and the expected unitthroughput TP_(PUE1) may be used as Shannon capacity in case of theSINR=−bias [dB].

The division ratio determination unit 123 determines the resourcedivision ratio by using Equation (2) on the basis of the number #UEperMof connected terminals and the number #UEperP of connected terminalsacquired by the information acquisition unit 122, and the expected unitthroughput TP_(MUE), TP_(PUE1), and TP_(PUE2) acquired by theinformation acquisition unit 122. In addition, in order to cope with achange in a communication situation, it is preferable that the divisionratio determination unit 123 periodically updates the resource divisionratio.

The resource assignment unit 124 assigns, to the radio terminal MUE, aradio resource (a resource block) of the macro cell usable PDSCHresources determined according to the resource division ratio determinedby the division ratio determination unit 123, and Equation (3). Forexample, on the basis of the CQI that is fed back from the radioterminal MUE, the resource assignment unit 124 assigns the radioresource (the resource block) of the macro cell usable PDSCH resourcesto the radio terminal MUE using a scheduling algorithm such asproportional fairness (PF).

(4) Configuration of Pico Cell Base Station

Next, the configuration of the pico cell base station PeNB will bedescribed. FIG. 6 is a block diagram illustrating the configuration ofthe pico cell base station PeNB according to the first embodiment.

As illustrated in FIG. 6, the pico cell base station PeNB includes anantenna 201, a radio communication unit 210, a control unit 220, astorage unit 230, and a network communication unit 240.

The radio communication unit 210 is configured, for example, using aradio frequency (RF) circuit, a baseband (BB) circuit and the like, andperforms transmission/reception of a radio signal with the radioterminal PUE through the antenna 201. Furthermore, the radiocommunication unit 210 performs modulation of a transmission signal anddemodulation of a reception signal.

The control unit 220 is configured, for example, using a CPU, andcontrols various functions provided in the pico cell base station PeNB.The storage unit 230 is configured, for example, using a memory, andstores various types of information used for the control and the like ofthe pico cell base station PeNB. The network communication unit 240performs inter-base station communication with another base stationusing the X2 interface.

The control unit 220 includes a connection destination selection unit221, an information generation unit 222, and a resource assignment unit223.

The connection destination selection unit 221 selects a base station,which is a next connection destination of a radio terminal PUE, on thebasis of RSRP reported from the radio terminal PUE connected to the picocell base station PeNB. In the case in which the radio terminal PUEreceives a reference signal from each of the macro cell base stationMeNB and the pico cell base station PeNB, when the RSRP_(MeNB)corresponding to the macro cell base station MeNB is compared with theRSRP_(PeNB) corresponding to the pico cell base station PeNB, theconnection destination selection unit 221 applies bias to theRSRP_(PeNB). When the biased RSRP_(PeNB) is lower than the RSRP_(MeNB),the connection destination selection unit 221 performs handover controlsuch that a connection destination of the radio terminal PUE is switchedto the macro cell base station MeNB.

The information generation unit 222 generates connection terminal numberinformation indicating the number of radio terminals PUE (the number#UEperP of connected terminals), which are connected to the pico cellbase station PeNB, by using information managed by the resourceassignment unit 223. The connection terminal number informationgenerated by the information generation unit 222 is transmitted to themacro cell base station MeNB from the network communication unit 240.

The resource assignment unit 223 assigns a radio resource (a resourceblock) to the radio terminal PUE. For example, on the basis of the CQIthat is fed back from the radio terminal PUE, the resource assignmentunit 223 assigns the radio resource (the resource block) of the PDSCHresources to the radio terminal PUE using a scheduling algorithm such asproportional fairness (PF).

(5) Operation of Radio Communication System

FIG. 7 is an operation sequence diagram illustrating the operation ofthe radio communication system 1 according to the first embodiment. Thesequence illustrated in FIG. 7 is periodically performed.

As illustrated in FIG. 7, in step S11 a, the information generation unit222 of the pico cell base station PeNB1 generates connection terminalnumber information indicating the number of radio terminals PUEconnected to the pico cell base station PeNB1. In step S12 a, thenetwork communication unit 240 of the pico cell base station PeNB1transmits the connection terminal number information to the macro cellbase station MeNB. The network communication unit 140 of the macro cellbase station MeNB receives the connection terminal number information.

In step S11 b, the information generation unit 222 of the pico cell basestation PeNB2 generates connection terminal number informationindicating the number of radio terminals PUE connected to the pico cellbase station PeNB2. In step S12 b, the network communication unit 240 ofthe pico cell base station PeNB2 transmits the connection terminalnumber information to the macro cell base station MeNB. The networkcommunication unit 140 of the macro cell base station MeNB receives theconnection terminal number information.

In step S11 c, the information generation unit 222 of the pico cell basestation PeNB3 generates connection terminal number informationindicating the number of radio terminals PUE connected to the pico cellbase station PeNB3. In step S12 c, the network communication unit 240 ofthe pico cell base station PeNB3 transmits the connection terminalnumber information to the macro cell base station MeNB. The networkcommunication unit 140 of the macro cell base station MeNB receives theconnection terminal number information.

In step S13, the information acquisition unit 122 of the macro cell basestation MeNB acquires the number of terminals connected to each of thepico cell base stations PeNB1 to PeNB3 on the basis of the connectionterminal number information received in the network communication unit140, and acquires an average value or a maximum value of the number ofterminals connected to each of the pico cell base stations PeNB1 toPeNB3 as the number #UEperP of connected terminals. Furthermore, theinformation acquisition unit 122 acquires the number #UEperM ofterminals connected to the macro cell base station MeNB. Moreover, theinformation acquisition unit 122 acquires the expected unit throughputTP_(MUE) and TP_(PUE2) stored in the storage unit 130, and acquires theexpected unit throughput TP_(PUE1) on the basis of the bias value storedin the storage unit 130.

In step S14, the division ratio determination unit 123 of the macro cellbase station MeNB determines the resource division ratio using Equation(2) on the basis of the number #UEperM of connected terminals and thenumber #UEperP of connected terminals acquired by the informationacquisition unit 122, and the expected unit throughput TP_(MUE),TP_(PUE1), and TP_(PUE2) acquired by the information acquisition unit122.

In step S15, the resource assignment unit 124 of the macro cell basestation MeNB assigns, to the radio terminal MUE, the radio resource (theresource block) of the macro cell usable PDSCH resources determinedaccording to the resource division ratio determined by the divisionratio determination unit 123, and Equation (3).

In step S16 a, the resource assignment unit 223 of the pico cell basestation PeNB1 assigns the radio resource (the resource block) to theradio terminal PUE connected to the pico cell base station PeNB1.Furthermore, since the macro cell unusable PDSCH resources do notreceive interference from the macro cell base station MeNB, the resourceassignment unit 223 assigns the macro cell unusable PDSCH resources tothe pico cell-side deteriorated terminal PUE connected to the pico cellbase station PeNB1. In addition, since the radio terminal PUEperiodically feeds back CQI to the pico cell base station PeNB1, thepico cell base station PeNB1 is able to preferentially assign the macrocell unusable PDSCH resource to the pico cell-side deteriorated terminalPUE according to the fact that the CQI is good.

In step S16 b, the resource assignment unit 223 of the pico cell basestation PeNB2 assigns the radio resource (the resource block) to theradio terminal PUE connected to the pico cell base station PeNB2.Furthermore, since the macro cell unusable PDSCH resources do notreceive interference from the macro cell base station MeNB, the resourceassignment unit 223 preferentially assigns the macro cell unusable PDSCHresources to the pico cell-side deteriorated terminal PUE connected tothe pico cell base station PeNB2.

In step S16 c, the resource assignment unit 223 of the pico cell basestation PeNB3 assigns the radio resource (the resource block) to theradio terminal PUE connected to the pico cell base station PeNB3.Furthermore, since the macro cell unusable PDSCH resources do notreceive interference from the macro cell base station MeNB, the resourceassignment unit 223 preferentially assigns the macro cell unusable PDSCHresources to the pico cell-side deteriorated terminal PUE connected tothe pico cell base station PeNB3.

(6) Effect of First Embodiment

As described above, in accordance with the radio communication system 1according to the first embodiment, with respect to radio resources to beused as the PDSCH by the macro cell base station MeNB, the macro cellusable PDSCH resources and the macro cell unusable PDSCH resources areprovided. Since the macro cell unusable PDSCH resources do not receiveinterference from the macro cell base station MeNB, the macro cellunusable PDSCH resources are preferentially assigned to the picocell-side deteriorated terminal PUE, resulting in the improvement of thethroughput of the pico cell-side deteriorated terminal PUE. Furthermore,the resource division ratio is determined according to the throughput ofthe macro cell-side deteriorated terminal MUE and the throughput of thepico cell-side deteriorated terminal PUE, so that it is possible toequalize the throughput of deteriorated terminals (cell edge terminals)of each of the macro cell base station MeNB and the pico cell basestation PeNB, resulting in the improvement of the throughput of theentire system.

In the first embodiment, since the division ratio determination unit 123determines the resource division ratio on the basis of an average valueor a maximum value of the number of connected terminals of each of aplurality of pico cell base stations PeNB, even when the plurality ofpico cell base stations PeNB are provided in the communication area ofthe macro cell base station MeNB, it is possible to appropriately setthe resource division ratio.

Second Embodiment

In the first embodiment, the macro cell base station MeNB determines theresource division ratio. However, in the second embodiment, the picocell base station PeNB determines the resource division ratio.Hereinafter, differences from the first embodiment will be described anda redundant description will be omitted.

FIG. 8 is a block diagram illustrating the configuration of the macrocell base station MeNB according to the second embodiment.

As illustrated in FIG. 8, the macro cell base station MeNB according tothe second embodiment includes an information generation unit 125, anddoes not include the information acquisition unit 122 and the divisionratio determination unit 123 described in the first embodiment. Theinformation generation unit 125 generates connection terminal numberinformation (information on the number of radio terminals MUE connectedto the macro cell base station MeNB).

FIG. 9 is a block diagram illustrating the configuration of the picocell base station PeNB according to the second embodiment.

As illustrated in FIG. 9, the pico cell base station PeNB according tothe second embodiment includes an information acquisition unit 224 and adivision ratio determination unit 225. The information acquisition unit224 acquires connection terminal number information of each of the macrocell base station MeNB and the pico cell base station PeNB. The divisionratio determination unit 225 determines a resource division ratio usingthe same method as that of the first embodiment on the basis of theconnection terminal number information of each of the macro cell basestation MeNB and the pico cell base station PeNB acquired by theinformation acquisition unit 224.

FIG. 10 is an operation sequence diagram illustrating the operation ofthe radio communication system 1 according to the second embodiment. Theoperation sequence illustrated in FIG. 10 is periodically performed.

In step S21 a, the information generation unit 125 of the macro cellbase station MeNB generates the connection terminal number informationindicating the number of radio terminals MUE connected to the macro cellbase station MeNB. In step S22 a, the network communication unit 140 ofthe macro cell base station MeNB transmits the connection terminalnumber information to the pico cell base station PeNB1. The networkcommunication unit 240 of the pico cell base station PeNB1 receives theconnection terminal number information.

In step S21 b, the information generation unit 222 of the pico cell basestation PeNB2 generates the connection terminal number informationindicating the number of radio terminals PUE connected to the pico cellbase station PeNB2. In step S22 b, the network communication unit 240 ofthe pico cell base station PeNB2 transmits the connection terminalnumber information to the pico cell base station PeNB1. The networkcommunication unit 240 of the pico cell base station PeNB1 receives theconnection terminal number information.

In step S21 c, the information generation unit 222 of the pico cell basestation PeNB3 generates the connection terminal number informationindicating the number of radio terminals PUE connected to the pico cellbase station PeNB3. In step S22 c, the network communication unit 240 ofthe pico cell base station PeNB3 transmits the connection terminalnumber information to the pico cell base station PeNB1. The networkcommunication unit 240 of the pico cell base station PeNB1 receives theconnection terminal number information.

In step S23, the information acquisition unit 224 of the pico cell basestation PeNB1 acquires an average value or a maximum value of the numberof terminals connected to each of the pico cell base stations PeNB1 toPeNB3 as the number #UEperP of connected terminals, and acquires thenumber #UEperM of terminals connected to the macro cell base stationMeNB. Moreover, the information acquisition unit 224 acquires expectedunit throughput TP_(MUE) and TP_(PUE2) stored in the storage unit 230,and acquires expected unit throughput T_(PUE1) on the basis of a biasvalue stored in the storage unit 230.

In step S24, the division ratio determination unit 225 of the pico cellbase station PeNB1 determines the resource division ratio using Equation(2) on the basis of the number #UEperM of connected terminals and thenumber #UEperP of connected terminals acquired by the informationacquisition unit 224, and the expected unit throughput TP_(MUE),TP_(PUE1), and TP_(PUE2) acquired by the information acquisition unit224.

In step S25, the network communication unit 240 of the pico cell basestation PeNB1 transmits information, which indicates the resourcedivision ratio determined by the division ratio determination unit 225,to the macro cell base station MeNB. The network communication unit 140of the macro cell base station MeNB receives the information indicatingthe resource division ratio.

Then, the resource assignment unit 124 of the macro cell base stationMeNB assigns, to the radio terminal MUE, a radio resource (a resourceblock) of the macro cell usable PDSCH resources determined according tothe information received in the network communication unit 140 andindicating the resource division ratio.

As described above, according to the second embodiment, it is possibleto obtain the same effects as those of the first embodiment.

Third Embodiment

In the first embodiment and the second embodiment, the PDSCH resourcesof the macro cell base station MeNB are frequency-divided into the macrocell unusable PDSCH resources unusable by the macro cell base stationMeNB and the macro cell usable PDSCH resources usable by the macro cellbase station MeNB.

In the third embodiment, the PDSCH resources of the macro cell basestation MeNB are frequency-divided into macro cell low power PDSCHresources and macro cell normal power PDSCH resources. The macro celllow power PDSCH resources are usable by the macro cell base stationMeNB, but are limited to have transmission power lower than that of themacro cell normal power PDSCH resources. In the third embodiment, themacro cell normal power PDSCH resources correspond to first radioresources and the macro cell low power PDSCH resources correspond tosecond radio resources.

FIG. 11 is a diagram for explaining ICIC according to the thirdembodiment. Mainly, differences from the first embodiment will bedescribed.

As illustrated in FIG. 11, in the third embodiment, the macro cellnormal power PDSCH resources are at least a part of all resource blocksof a downlink, and the macro cell low power PDSCH resources are theremaining resource blocks obtained of all the resource blocks of thedownlink, except for the partial resource blocks.

Since the macro cell low power PDSCH resources receive low levelinterference from the macro cell base station MeNB, the pico cell basestation PeNB assigns the PDSCH resources to a radio terminal PUE with alow SINR. Since the radio terminal PUE periodically feeds back ameasurement result of reception quality to the pico cell base stationPeNB as channel quality information (CQI), the pico cell base stationPeNB is able to preferentially assign low interference PDSCH resourcesto the radio terminal PUE in response to the fact that CQI correspondingto the low interference PDSCH resources is good.

Furthermore, it is preferable that the macro cell base station MeNBassigns the macro cell low power PDSCH resources to a radio terminal MUEin the vicinity of the macro cell base station MeNB. Specifically, theresource assignment unit 124 of the macro cell base station MeNB assignsa radio resource (a resource block) of the macro cell low power PDSCHresources to a radio terminal MUE with good CQI corresponding to themacro cell low power PDSCH resources or a radio terminal MUE with smallpath loss between the radio terminal MUE and the macro cell base stationMeNB. The resource assignment unit 124 assigns a radio resource (aresource block) of the macro cell normal power PDSCH resources to aradio terminal MUE with bad CQI corresponding to the macro cell lowpower PDSCH resources or a radio terminal MUE with large path lossbetween the radio terminal MUE and the macro cell base station MeNB.

In the third embodiment, similarly to the first embodiment, a resourcedivision ratio, which indicates a ratio of the macro cell low powerPDSCH resources and the macro cell normal power PDSCH resources, isdetermined using Equation (2). Furthermore, similarly to the firstembodiment, the resource division ratio matches the resolution of theCQI that is fed back.

As described above, according to the third embodiment, the interferencereduction effect of the pico cell base station PeNB is lower than thefirst embodiment. However, since PDSCH resources usable by the macrocell base station MeNB are increased as compared with the firstembodiment, it is possible to improve the throughput of the macro cellbase station MeNB.

Other Embodiments

As described above, the present invention has been described accordingto the embodiments. However, it must not be understood that thediscussions and the drawings constituting a part of this disclosurelimit the present invention/From this disclosure, various alternativeembodiments, examples and operational techniques are apparent to thoseskilled in the art.

In each of the aforementioned each embodiments, the case offrequency-dividing the PDSCH resources has been described. However, thePDSCH resources may be time-divided. FIG. 12 is a diagram for explainingthe case of time-dividing the PDSCH resources. As illustrated in FIG.12, a data region of a downlink subframe is time-divided to provide themacro cell unusable PDSCH resources (or the macro cell low power PDSCHresources) and the macro cell usable PDSCH resources (or the macro cellnormal power PDSCH resources). A ratio of the time division may bearbitrarily set. However, according to the LTE specifications, theresources are divided in the units of OFDM symbols.

Alternatively, the subframe may not be time-divided in the units of OFDMsymbols, and the radio frame illustrated in FIG. 2 may be time-dividedin the units of subframes. FIG. 13 is a diagram for explaining the caseof time-dividing a radio frame in the units of subframes. As illustratedin FIG. 13, one radio frame includes subframes corresponding to themacro cell unusable PDSCH resources (or the macro cell low power PDSCHresources) and subframes corresponding to the macro cell usable PDSCHresources (or the macro cell normal power PDSCH resources).

In each of the aforementioned embodiments, the resource division onPDSCH (that is, the data region division) has been described. However,the present invention may be applied not only to the PDSCH, but also toresource division on PDCCH (that is, control region division). For theresource division on PDCCH, frequency division or time division may beemployed.

In each of the aforementioned embodiments, the case has been describedin which the coverage of the pico cell base station PeNB is expanded.However, the present invention is not limited thereto. Even when thecoverage of the pico cell base station PeNB is not expanded, the presentinvention is effective in the reduction of inter-base stationinterference in the heterogeneous network.

In each of the aforementioned embodiments, the load level indicates thenumber of radio terminals in a connected state. However, in addition tosuch a load level index, a use rate of a radio resource or the amount ofpackets transmitted and received, for example, may be employed as theload level.

In addition, in LTE Advanced, since a relay node, that is, a basestation in which backhaul is configured by radio, is expected to beemployed, and the X2 interface is expected to be employed in the relaynode, the relay node may be used as the low power base station accordingto the present invention.

Moreover, in each of the aforementioned embodiments, the LTE system hasbeen described. However, the present invention may also be applied toother radio communication systems such as radio communication systemsbased on WiMAX (IEEE 802.16).

As described above, it must be understood that the present inventionincludes various embodiments and the like that are not described herein.

Note that the entire content of the Japanese Patent Application No.2010-277383 (filed on Dec. 13, 2010) is incorporated herein byreference.

INDUSTRIAL APPLICABILITY

As mentioned above, a radio communication system, a base station, and acontrol method of the base station of the present invention is usefulfor mobile communication, with which it is possible to reduce inter-basestation interference in a heterogeneous network and to improve thethroughput of the entire system.

The invention claimed is:
 1. A radio communication system comprising: ahigh power base station; and a low power base station provided in acommunication area of the high power base station and havingtransmission power lower than transmission power of the high power basestation, comprising: a division ratio determination unit that determinesa resource division ratio with respect to radio resources to be used asa specific downlink channel by the high power base station, the resourcedivision ratio indicating a ratio of first radio resources and secondradio resources, the transmission power of the high power base stationfor second radio resources being limited to be lower than that for thefirst radio resources, wherein the division ratio determination unitdetermines the resource division ratio according to expected throughputof a high power-side deteriorated terminal having a deterioratedreception state, and expected throughput of a low power-sidedeteriorated terminal having a deteriorated reception state, the highpower-side deteriorated terminal being a radio terminal connected to thehigh power base station, the low power-side deteriorated terminal beinga radio terminal connected to the low power base station, wherein thedivision ratio determination unit determines a ratio of the first radioresource with respect to radio resources to be used as the specificdownlink channel based on values of a load level of the high power basestation, an expected unit throughput corresponding to the receptionquality level of a high power-side deteriorated terminal, a load levelof the low power base station, an expected unit throughput correspondingto the reception quality level of a first low power-side deterioratedterminal and an expected unit throughput corresponding to the receptionquality level of a second low power-side deteriorated terminal.
 2. Theradio communication system according to claim 1, wherein the divisionratio determination unit determines the resource division ratio suchthat the expected throughput of the high power-side deterioratedterminal is equal to the expected throughput of the low power-sidedeteriorated terminal.
 3. The radio communication system according toclaim 1, wherein the expected throughput of the high power-sidedeteriorated terminal is determined on the basis of a load level of thehigh power base station and expected unit throughput corresponding to areception quality level of the high power-side deteriorated terminal,and the expected throughput of the low power-side deteriorated terminalis determined on the basis of a load level of the low power base stationand expected unit throughout corresponding to a reception quality levelof the low power-side deteriorated terminal.
 4. The radio communicationsystem according to claim 3, wherein the load level of the high powerbase station indicates the number of radio terminals connected to thehigh power base station, and the load level of the low power basestation indicates the number of radio terminals connected to the lowpower base station.
 5. The radio communication system according to claim3, wherein the low power-side deteriorated terminal includes a first lowpower-side deteriorated terminal to which the first radio resource isassigned, and a second low power-side deteriorated terminal to which thesecond radio resource is assigned, and throughput of the low power-sidedeteriorated terminal is determined on the basis of the load level ofthe low power base station, expected unit throughput corresponding to areception quality level of the first low power-side deterioratedterminal, and expected unit throughput corresponding to a receptionquality level of the second low power-side deteriorated terminal.
 6. Theradio communication system according to claim 5, wherein when the loadlevel of the high power base station is set as #UEperM, the expectedunit throughput corresponding to the reception quality level of the highpower-side deteriorated terminal is set as TPMuE, the load level of thelow power base station is set as #UEperP, the expected unit throughputcorresponding to the reception quality level of the first low power-sidedeteriorated terminal is set as TPpuE1, and the expected unit throughputcorresponding to the reception quality level of the second lowpower-side deteriorated terminal is set as TPpuE2, the division ratiodetermination unit determines a ratio alpha of the first radio resourcewith respect to radio resources to be used as the specific downlinkchannel by using the calculation equation:alpha=(TPpuE2(times)#UEperM)/(TPMuE(times)#UEperP+(TPpuE2−TPpuE1)(times)#UEperM.7. The radio communication system according to claim 1, wherein, when aplurality of the low power base stations are provided in thecommunication area of the high power base station, the division ratiodetermination unit uses an average value or a maximum value of loadlevels of the low power base stations as the load level of the low powerbase station.
 8. The radio communication system according to claim 1,further comprising: a resource assignment unit that assigns a radioresource to a radio terminal connected to the high power base station,wherein the first radio resource is a radio resource usable by the highpower base station, the second radio resource is a radio resourceunusable by the high power base station, and the resource assignmentunit assigns a radio resource of the first radio resources, which aredetermined according to the resource division ratio determined by thedivision ratio determination unit, to the radio terminal connected tothe high power base station.
 9. The radio communication system accordingto claim 1, further comprising: a resource assignment unit that assignsa radio resource to a radio terminal connected to the high power basestation, wherein the first radio resource is a radio resource in whichtransmission power of the high power base station is not limited, thesecond radio resource is a radio resource in which the transmissionpower of the high power base station is limited, and the resourceassignment unit assigns a radio resource of the first radio resourcesand the second radio resources, which are determined according to theresource division ratio determined by the division ratio determinationunit, to the radio terminal connected to the high power base station.10. The radio communication system according to claim 1, wherein thespecific downlink channel is a data transmission channel fortransmitting user data to the radio terminal.
 11. The radiocommunication system according to claim 1, wherein the second radioresources are at least a part of frequency bands of all frequency bandsof a downlink, and the first radio resources correspond to remainingfrequency bands of all the frequency bands of the downlink except forthe partial frequency bands.
 12. The radio communication systemaccording to claim 11, wherein each of the partial frequency bands andthe remaining frequency bands is an integer times of a frequency unit bywhich the radio terminal measures a reception quality level.
 13. Theradio communication system according to claim 1, wherein the secondradio resources are at least a part of a time range of a data region fortransmitting user data to the radio terminal in a communication timeframe of a downlink, and the first radio resources are a remaining timerange of the data region except for the part of a time range.
 14. A basestation comprising: a division ratio determination unit that determinesa resource division ratio with respect to radio resources to be used asa specific downlink channel by a high power base station, the resourcedivision ratio indicating a ratio of first radio resources and secondradio resources, the transmission power of the high power base stationfor the second radio resources being limited to be lower than that forthe first radio resources, wherein the division ratio determination unitdetermines the resource division ratio according to expected throughputof a deteriorated terminal being a radio terminal connected to the basestation and having a deteriorated reception state, and expectedthroughput of a deteriorated terminal being a radio terminal connectedto a neighboring base station and having a deteriorated reception state,wherein the division ratio determination unit determines a ratio of thefirst radio resource with respect to radio resources to be used as thespecific downlink channel based on values of a load level of the highpower base station, an expected unit throughput corresponding to thereception quality level of a high power-side deteriorated terminal, aload level of a low power base station that is provided in acommunication area of the high power base station, an expected unitthroughput corresponding to the reception quality level of a first lowpower-side deteriorated terminal and an expected unit throughputcorresponding to the reception quality level of a second low power-sidedeteriorated terminal.
 15. A control method of a base station,comprising: a step of determining a resource division ratio with respectto radio resources to be used as a specific downlink channel by a highpower base station, the resource division ratio indicating a ratio offirst radio resources and second radio resources, the transmission powerof the high power base station for the second radio resources beinglimited to be lower than that for the first radio resources, wherein inthe step of determining, the resource division ratio is determinedaccording to expected throughput of a deteriorated terminal being aradio terminal connected to the base station and having a deterioratedreception state, and expected throughput of a deteriorated terminalbeing a radio terminal connected to a neighboring base station andhaving a deteriorated reception state, and determining a ratio of thefirst radio resource with respect to radio resources to be used as thespecific downlink channel based on values of a load level of a higherpower base station, an expected unit throughput corresponding to thereception quality level of a high power-side deteriorated terminal, aload level of a low power base station that is provided in acommunication area of the high power base station, an expected unitthroughput corresponding to the reception quality level of a first lowpower-side deteriorated terminal and an expected unit throughputcorresponding to the reception quality level of a second low power-sidedeteriorated terminal.